In Article " Conductor Ampacity Calculation – Part Four " I explained the rule that controlling and refining the Conductor Ampacity selection from NEC Tables.
Also, in Article " Conductor Ampacity Calculation – Part Five ", I explained Correction/ adjustment Factors Affecting Conductor Ampacity Tables as provided in 310.15(B)
Today, I will explain some special cases for conductor ampacity calculations as follows.
For more information and good following, please review the following articles:
Special Cases for Conductor Ampacity Calculations
 Bare or Covered Conductors,
 Neutral Conductor,
 Grounding or Bonding Conductor,
 Main feeders to Dwelling units,
 Conductors contained in sheet metal auxiliary gutter,
 Conductors contained in Nonmetallic Auxiliary Gutters,
 Conductors in Metal Wireways,
 Conductors for Remote RemoteControl, Signaling, And Powerlimited Circuits,
 Conductors for Fire Alarm Systems,
 Conductors and Cables in Cable Trays.
Case#1: Bare or Covered
Conductors
Where
bare or covered conductors are installed with insulated conductors, the
temperature rating of the bare or covered conductor shall be equal to the
lowest temperature rating of the insulated conductors for the purpose of
determining ampacity.

Case#2: Neutral Conductor

Neutral is a
current carrying conductor – apply table 310.15(B)(3)(a)

Neutral is not a
current carrying conductor – don’t apply table 310.15(B)(3)(a)

In circuits having a 3wire circuit from a 4wire, 3phase,
208Y/120V or 480Y/277V (see fig.1).

If Neutral carries only the unbalanced current from other
conductors of the same circuit.

In balanced 4wire, 3phase, circuit that has nonlinear loads
exceeds 50% of its loads (see fig.2).

In Balanced circuits like 3 wire, 1phase, 120/240 V or 4wire,
3Phase, 208Y/120V or 480Y/277V (see fig.3).

Fig (1)

Definition:
Nonlinear Load: A load where the wave shape of the
steadystate current does not follow the wave shape of the applied voltage.

Example#1:
Solution:
Step#2: load operating for 3.5 hours, so it will be a continuous load
The THHN Conductor is sized for Continuous current loading = 83.3 X 125% = 104.125 Amps
Step#3: application of Adjustment factors
Since more than 50% 0f the load is nonlinear loads (fluorescent lighting), so the neutral will be a current carrying conductor – apply table 310.15(B)(3)(a)
From Table 31015(b)(3)(a) and for (46 conductors), the adjustment factor = 80%
From Table 31015(b)(2)(a) and for an ambient temperature 65° C and temperature rating of conductor 90° C , the ambient temperature correction factor = 0.65
Step#4: Get Wire corrected Ampacity
Corrected Ampacity = 104.125 / (0.8 x 0.65) = 200.24 amps
Step 4: Get suitable Wire size
From table 310.15(B)(16) , for THHN Conductors but for 75° C terminations, we will use 75° C Column.
You find the suitable conductor size is 4/0 AWG.
Case#3: Grounding or
Bonding Conductor
A
grounding or bonding conductor shall not be considered current carrying conductor
and not counted when applying the adjustment factor of table 310.15(B)(3)(a) where is More Than
Three CurrentCarrying Conductors in a Raceway or Cable.

Case#4: Main feeders to Dwelling units
120/240volt,
3wire, singlephase serviceentrance conductors, service lateral conductors,
and feeder conductors that serve as the main power feeder to each dwelling
unit and are installed in raceway or cable with or without an equipment
grounding conductor. These conductors can be sized according to Table
310.15(B)(7) with condition that it carries 100 percent of the dwelling
unit’s diversified load.

Notes to Case#4:

Example#2:
What size service/feeder conductors would be required if the total adjusted demand load for a dwelling unit equals 195 A and the maximum unbalanced load is 70A?
Solution:
Step#1: ungrounded service/feeder conductors size
From Table 310.15(B)(7) , and for adjusted demand load for a dwelling unit 195 A
The Feeder size will be 2/0 AWG rated 200 A.
Step#2: the Grounded (neutral) conductor size
250.24(B) requires the grounded (neutral) conductor to be sized no smaller than 4 AWG based on table 250.66
Since the maximum unbalanced load is 70A and from table 310.15(B)(16), 75° C column
The Grounded (neutral) conductor size will be 4 AWG, rated 85 A at 75° C.
Case#5: Conductors contained in sheet metal auxiliary gutter
As per NEC 366.23(A),
Where the number of currentcarrying conductors contained in the sheet metal
auxiliary gutter is 30 or less, the adjustment factors specified in Table
310.15(B)(3)(a) shall not apply.

Important!!!
As per NEC 366.23(A):

Case#6: Conductors contained in Nonmetallic Auxiliary Gutters
As per NEC 366.23(B),
The adjustment factors specified in 310.15(B)(3)(a) shall be applicable to
the current carrying conductors in the nonmetallic auxiliary gutter.

Case#7: Conductors In Metal Wireways
As per 376.22(B),
the conductor ampacity requirements will be as follows:

Important!!!
As per
376.22(A), the number of conductors in
metal wireways will be determined as follows:

Case#8: Conductors For Remote RemoteControl, Signaling, And
Powerlimited Circuits
As per 725.51, the
conductor ampacity requirements will be as follows:
(A) Where Class 1
circuit conductors are permitted in a raceway, The ampacity adjustment
factors given in 310.15(B)(3)(a) shall apply only if such conductors carry
continuous loads in excess of 10 percent of the ampacity of each conductor.
(B) Where
powersupply conductors and Class 1 circuit conductors are permitted in a
raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall apply
as follows:

Important!!!
Class 1 circuits:
The portion of the wiring system between the load side of the overcurrent
device or powerlimited supply and the connected equipment.
Class 1 circuits
shall be classified as follows:

Case#9: Conductors for Fire Alarm Systems
As per 760.51, the
conductor ampacity requirements will be as follows:
(A) Where only
non–powerlimited fire alarm circuit and Class 1 circuit conductors are in a
raceway, The ampacity adjustment factors given in 310.15(B)(3)(a) shall apply
if such conductors carry continuous load in excess of 10 percent of the
ampacity of each conductor.
(B) Where powersupply
conductors and fire alarm circuit conductors are permitted in a raceway, The
ampacity adjustment factors given in 310.15(B)(3)(a) shall apply as follows:
(C) As per 760.130, Where
only non–powerlimited fire alarm circuit conductors are in a raceway, The
ampacity adjustment factors given in 310.15(B)(3)(a) shall not apply.

In the next Article, I will explain the Last Special Cases for Conductor Ampacity Calculations: Conductors and Cables in Cable Trays. Please, keep following.
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